A typical blowout preventer 10 is shown in FIG. 1 through 3. The blowout preventer 10 has a body 12, piston rod 14 controlled by operators 15, a central bore 16 for receiving a tubular member (not shown), and a removable pressure plate 18 that covers an access opening into the inner cavity of the body 12. The depicted blowout preventer is known as a “double gate”, which has four hydraulic operators 12, two for each “gate” 14. Other configurations, not shown, include a single gate with two operators, a triple gate with six operators and so on.
Referring to FIG. 4 through 7, a typical hydraulic operator 15 for a ram type blowout preventer with a lock screw 20 is shown. FIG. 4 shows the hydraulic operator 15 in the open position with the lock screw 20 in the open position, FIG. 5 shows the hydraulic operator 15 in the partially closed position with the lock screw 20 in the open position, FIG. 6 shows the hydraulic operator 15 in the closed position with the lock screw 20 in the open position, and FIG. 7 shows the hydraulic operator 15 in the fully closed position with the lock screw 20 in the closed position.
In this description, the ram shaft and piston shaft are referred to by the generic term piston rod 14. In the depicted embodiment, the piston 22 is threaded and sealed onto the piston rod 14. When the piston 22 is assembled into position, setscrews (not shown) are generally installed to secure it in place. The setscrews are torqued to “dig” into the piston rod 14 and cause permanent deformation of the piston rod material. The end 24 of the piston rod 14 is turned to a smaller diameter to receive the ram block (not shown) and does not require a specific orientation. The piston 22 is locked into position with the setscrews and turns with the ram shaft if the ram shaft turns in any of its functions.
Referring to FIG. 6 the operator 15 is in the fully closed position and the lock screw 20 is in the open position. The lock screw 20 is threaded into the end of the bonnet 26. To lock the operator in the closed position, the lock screw 20 is rotated until it contacts the end of the piston rod 14 as shown in FIG. 7.
There are two problems associated with the type of operator system shown in FIG. 4 through 7. Firstly, the setscrews cause permanent damage to the piston rod in the threaded area for the piston attachment and positioning. Service and repair of this system can be difficult and costly if the threads are too badly damaged by the setscrews. Secondly, the lock screw is threaded into the end of the “end cover”. Although there is a “cover” attached to the end of the “end cover”, the threads of both the “end cover” and mating “lock screw” are subject to wear from abrasive fluid contamination. Dust, dirt and particle laden well fluids contaminate the mating threads and cause deterioration in use.
Referring now to FIG. 8 through 11, a second type of hydraulic operator 15 is depicted. FIG. 8 shows the hydraulic operator 15 in the open position with the lock screw 20 in the open position, FIG. 9 shows the hydraulic operator 15 in the partially closed position with the lock screw 20 in the open position, FIG. 10 shows the hydraulic operator 15 in the closed position with the lock screw 20 in the open position, and FIG. 11 shows the hydraulic operator 15 in the fully closed position with the lock screw 20 in the closed position.
In this type of operator 15, the threads of the lock screw 20 are contained in the hydraulic fluid assembly and fully protected from abrasive fluids. Although this system is effective in protecting the mating threads, it introduces the problem of fluid displacement within the operator 15 when the lock screw 20 is engaged or disengaged. Fluid displacement within a hydraulic chamber 28 by mechanical means can be extremely dangerous. If the lock screw has been engaged for a period of time, over night for example, and needs to be released the next morning to resume rig operations, it is most important to ensure that there is a means to allow the fluid to flow freely out of the operating chamber 28 as the lock screw is screwed in. If personnel forget to attach the hydraulic hoses to their connections, or a connection fails, blocking the free flow of fluid out of the operating chamber 28 when disengaging the lock screw 20, extreme pressures can develop within the operating chamber 28. On more than one occasion, pressures have reached magnitudes that have caused the failure of the 4 retaining bolts that attach the “threaded nut” to the “piston”. The situation is hazardous because the failure of the four retaining bolts is catastrophic and releases the stored energy from the pressurized fluid instantaneously. The wrench and attachments used to turn the lock screw 20 can become projectiles when failure occurs.